The corrosion and deterioration of reinforcing steel in concrete which is exposed to ambient conditions promoting oxidation is a well-known problem in structural fields, and the problem becomes even more acute when the steel is exposed to chloride conditions that tend to promote oxidation to an adnormal extent, as, for example, when the steel is exposed to deicing salt, coastal atmospheric conditions, tidal wetting or flowing sea water. Thus, it is common for steel reinforcing bars used in structural concrete to be exposed to one or more of the foregoing adverse conditions, and spalling and delamenation of structural concrete resulting from the corrosion of the reinforcing steel is the major cause of deterioration of concrete structures subjected to deicing salts (e.g. highway bridge structures) or to marine environment chlorides (e.g. structures having structural concrete pillars located at least partially submerged in salt water).
In an effort to inhibit the corrosion of steel by oxidation, efforts have been made to apply a suitable protective coating to steel members which serves to prevent direct contact between the steel and the corrosion producing elements. For example, it is common practice to galvanize steel to increase its service life, and while galvanized steel has the advantage of being relatively inexpensive and providing generally satisfactory results under normal ambient conditions, it has been found that galvanized steel has a relatively rapid rate of oxidation when the steel is situated in a structural concrete environment so that it is generally unsatisfactory for uses of this sort.
Because of the generally unsatisfactory results obtained from galvanized steel used in structural concrete, some steel producers have applied an epoxy coating to steel reinforcing members. However, epoxy coatings are relatively expensive, and, perhaps more importantly, they are somewhat brittle and therefore subject to damage as a result of the normal wear and tear that is inherent in shipping and placing the epoxy coated reinforcing members in construction projects. When the relatively thin epoxy coating becomes cracked or pierced during shipping or handling, the steel core is exposed to galvanic corrosion which tends to spread rapidly beneath the epoxy coating.
The benefits of applying a protective coating of a corrosion-resistant metal, such as nickel, to a steel core are also well-known, but difficulties have been encountered in devising a method of applying nickel coatings to steel which is economically feasible and which is commercially practical in the sense that it can be utilized in conventional carbon steel bar mill operations using equipment already available.
At the present time, there are four principal methods used in the steel industry to coat steel with corrosion resistant metals such as nickel. One involves the conventional and well-known process of simply electro-depositing the plating nickel onto a steel base metal, such plating process resulting in the steel having a dentritic exterior surface formation which is less corrosion resistant than a wrought nickel formation typical of rolled or forged nickel. Secondly, it is known to cast molten nickel about a steel billet arranged in a mold, as disclosed, for example in Mudge U.S. Pat. No. 2,037,732. Another method involves the utilization of nickel in a vehicle which is applied to the steel, as by spraying, then heat treating the sprayed steel to drive off the vehicle, after which the steel is cold rolled, all as disclosed, for example, in Wesley U.S. Pat. No. 2,289,614, Freybergen U.S. Pat. No. 3,299,503 and Flint U.S. Pat. No. 3,316,625. Finally, a known plating process includes the steps of welding a nickel sheet to a steel plate, and then bonding the two metals using pressure rolls to reduce the thickness of the sandwiched metals.
Experimental work was conducted sometime ago on a relatively small scale to produce nickel coated steel reinforcing members, and while the end product resulting from such experimental work was found to be satisfactory, the method was not commercialized because it was not believed to be commercially feasible in carbon steel bar mill operations.
In the foregoing experimental work, a conventional steel billet having a generally rectangular cross-section was coated with a layer of nickel using conventional electroplating techniques, and the plated steel billet was then hot rolled to its final round shape. The resulting bar was coated with nickel, and had a wrought exterior surface that was preferable to the dentritic surface resulting from simply electroplating the nickel coating on the bar. Moreover, the hot rolling of the billet after it had been plated with nickel sometimes resulted in the formation of a nickel-iron alloy interface between the exterior nickel surface and the steel core, but it is not believed that the beneficial protective characteristics of this interface was recognized. One of the drawbacks of the resulting nickel coated bar was that the thickness of the wrought nickel surface layer was not entirely uniform, probably because the billet was square at the time the electroplating of the nickel was done, and subsequent hot rolling of the billet into a round shape did not uniformly distribute the nickel about the steel during rolling due to differential oxidation of the plated surface during the reheating.
Moreover, the foregoing experimental work was done on a small scale which did not take into consideration the practical aspects of commercially using the plating method in existing bar mill operations. For example, nickel is quite expensive and it is known that when nickel is exposed to oxygen at the high temperature required for hot rolling, significant quantities of nickel are passed off and lost through gas diffusion. This condition is conventionally corrected or alleviated by heating a nickel coated billet in equipment which provides a reducing atmosphere having little or no oxygen, but such equipment is relatively expensive and is not normally found in carbon steel bar mill operations. Additionally, when the nickel plated billets are heated to the above-mentioned high temperature, the nickel plating becomes somewhat soft and tacky, and if such billets are located closely adjacent to one another they tend to become welded to one another. While this problem is not too severe in small scale experimental work where the individual plated steel billets are heated separately or at least in spaced relation to one another, the economic considerations in a commercial operation dictate that a large number of billets be heated simultaneously in as small an area as possible (e.g. with the billets in abutment) to raise the production capacity of the process to an economically feasible level, and, in this regard, conventional steel billet furnaces presently found in carbon steel bar mill operations are designed to hold a large number of billets in close, abutting relation during heating.
In accordance with the present invention, a unique method of producing hot rolled nickel coated steel bars is provided which overcomes the draw backs of the prior art discussed above, and which provides a relatively inexpensive process for use in carbon steel bar mill operations using generally available conventional equipment in a commercially practiced operation.